In a supervisory control system where a large number of remote units transmit data to a central control, but each remote unit transmits only a small quantity of information, in such a system the length of the interrogation cycle takes longer than the meaningful information sent back by the remote units to the central control.
According to the prior art, in a system with a large number of remote units, the interrogation word is composed of a number of bits which define the address of the remote unit; and if it is wished to add reliability to the system, a number of error detection bits must also be added. In contrast, the number of information bits sent back from the remote is low.
If all the information bits sent back comprise status information such as whether a monitored unit is operating or not operating, which is easily indicated by binary bits zero and one, the conventional method becomes wasteful from the point of view of time because long questions or interrogations from the central control receive only short answers from the remote units, and thus most of the time is taken up by the questions, and not by the transmission of useful information back to central control.